1. Field of the Invention
This invention relates to methods and means for synthetic D.C. restoration of a.c. coupled electronic signals. Parallel scanning detector arrays, and particularly thermal imaging detector arrays are advantageously employed with this invention.
2. Prior Art
When an electric signal from a detector is a.c. coupled to an electrical processing system, the D.C. component in the signal is deleted. For a plurality of a.c. coupled detectors, the average value of the remaining a.c. component is the same for each detector.
A typical parallel scanning device 10 (see FIG. 1) includes a detector array 11 having a plurality of detectors 12 which are vertically disposed and which scan horizontally in unison across a field of view (FOV). Each of detectors 12 produces an electronic signal generated from scanning a separate line in the FOV. The scan time for each detector 12 is the same and detectors 12 scan simultaneously. The output from each detector 12 is a.c. coupled to a processing system 14. The output from processing system 14 affects an imaging device, such as video screen 15.
Scan line 18 includes three portions, 20, 22 and 24. Portions 20 and 22 correspond to background images whereas portion 24 corresponds to a hot spot. Since the average value 26 of scan line 16 and 18 is the same (average value 26 is set equal to zero in FIGS. 2 and 4), portions 20 and 22 of scan line 18 must be shifted or offset below average value 26 to compensate for hot spot 24. This shift causes scan line 18 to appear darker than it should be, resulting in a streak on screen 15. In order to compensate for this shift, it is necessary to estimate the shift and add it to scan line 18. This compensation process is called synthetic D.C. restoration.
Heretofore no effective system has been disclosed for easily estimating the shift of a.c. coupled signals from a parallel scanning detector system and for restoring the same. In particular, no analog electronic processing system has been proposed for providing synthetic D.C. restoration. Analog processing offers advantages over digital processing in that no analog/digital conversion is required, no complex histogramming is nec essary and, thus, real time implementation is facilitated.